WASHINGTON — Sen. Robert Menendez employed as an unpaid intern in his Senate office an illegal immigrant who was a registered sex offender, now under arrest by immigration authorities, The Associated Press has learned. The Homeland Security Department instructed federal agents not to arrest him until after Election Day, a U.S. official involved in the case told the AP.

A Homeland Security spokesman, Peter Boogaard, said Wednesday that it was “categorically false” that the department delayed the arrest of Luis Abrahan Sanchez Zavaleta, 18, until after the election.

Sanchez, an immigrant from Peru, was arrested by Immigration and Customs Enforcement agents in front of his home in New Jersey on Dec. 6, two federal officials said. Sanchez, who entered the country on a now-expired visitor visa from Peru, is facing deportation and remains in custody. The officials spoke on condition of anonymity because they were not authorized to discuss details of Sanchez’s immigration case.

Boogaard said in a statement that ICE followed standard procedures working with local prosecutors before taking what he described as appropriate enforcement action.

Menendez, D-N.J., who advocates aggressively for pro-immigration policies, was re-elected in November with 58 percent of the vote. He said his staff was notified about the case Monday, he personally learned about the case from AP’s reporting and knew nothing about whether DHS delayed the arrest. The senator said his staff asks interns whether they are in the country legally but cannot check to be sure.

“We certainly wouldn’t have known through any background checks since he is a minor about any sex offender status,” Menendez said in an interview Wednesday with MSNBC. “Once it came to our attention, our New Jersey staff director let the young man go.”

Online jail records did not indicate whether Sanchez has an attorney. Sanchez declined to be interviewed from jail.

The prosecutor’s office in Hudson County, N.J., said Sanchez was found to have violated the law in 2010 and subsequently required to register as a sex offender. The exact charge was unclear because Sanchez was prosecuted as a juvenile and those court records are not publicly accessible. The prosecutor’s office confirmed to AP that Sanchez registered as a sex offender, although his name does not appear on the public registry. The acting county prosecutor, Gaetano Gregory, is a Republican.

Authorities in Hudson County notified ICE agents in early October that they suspected Sanchez was an illegal immigrant who was a registered sex offender and who may be eligible to be deported. ICE agents in New Jersey notified superiors at the Homeland Security Department because they considered it a potentially high profile arrest, and DHS instructed them not to arrest Sanchez until after the November election, one U.S. official told the AP. ICE officials complained that the delay was inappropriate, but DHS directed them several times not to act, the official said.

It was not immediately clear why federal immigration authorities would not have been notified sooner about Sanchez’s status.

During discussions about when and where to arrest Sanchez, the U.S. reviewed Sanchez’s application for permission to stay in the country as part of President Barack Obama’s policy to allow up to 1.7 million young illegal immigrants avoid deportation and get permission to work for up to two years. As a sex offender, he would not have been eligible. Citizenship and Immigration Services, which oversees the program known as Deferred Action for Childhood Arrivals, notified Sanchez of that shortly before his arrest, one official said.

Menendez said the arrest spoke to the need for comprehensive immigration reform that brings illegal immigrants out of the shadows.

“It does speak volumes about why we need comprehensive immigration reform,” the senator said. “I can’t know who is here to pursue the American dream versus who is here to do it damage if I cannot get people to come forth out of the shadows, go through criminal background checks and then determine who is here to pursue the dream and make sure that those who are here and have criminal backgrounds ultimately get deported.”

During the final weeks of President George W. Bush’s administration, ICE was criticized for delaying the arrest of President Barack Obama’s aunt, who had ignored an immigration judge’s order to leave the country several years earlier after her asylum claim was denied. She subsequently won the right to stay in the United States after an earlier deportation order, and there was no evidence of involvement by the White House.

In that case, the Homeland Security Department had imposed an unusual directive days before the 2008 election requiring high-level approval before federal agents nationwide could arrest fugitive immigrants including Zeituni Onyango, the half-sister of Obama’s late father. The directive from ICE expressed concerns about “negative media or congressional interest,” according to a copy of that directive obtained by AP. The department lifted the immigration order weeks later.